Why FT8 is one of the WSJT-X weak-signal digital modes developed by Joe Taylor (K1JT), designed to complete a minimal contact (callsigns, grid squares, signal reports) in structured 15-second transmit/receive sequences. It uses 8-tone FSK in a bandwidth of only about 50 Hz, which concentrates the transmitted energy and lets the decoder pull signals out of the noise roughly 20 dB below what the ear can hear. That is why it is popular for HF DX and weak-signal VHF work with modest antennas and power.
Watch out The slow-scan TV description is a distractor built from real-sounding jargon; FT8 sends no images, just short structured data messages. The 8 in the name refers to the eight FSK tones, not to eight channels or a wide FM signal.
FT8 = Franke-Taylor, 8 tones, 15-second slots, decodes far below the noise floor.